Implement the phony assignment statement `_ = <expr>`.
<rdar://problem/108347427>
Pull request: https://github.com/WebKit/WebKit/pull/12999
Pull request: https://github.com/WebKit/WebKit/pull/13001
Committed 263268@main (c9231428cacb): <https://commits.webkit.org/263268@main> Reviewed commits have been landed. Closing PR #12999 and removing active labels.